1. Field of the Invention
The present invention relates to a mirror positioning structure that compensates for skew and bow, and a laser scanning unit employing the same. More particularly, the present invention relates to a structure that facilitates control of the rotation angle and the amount of rectilinear motion of a mirror that compensates for skew and bow, and a laser scanning unit employing the structure.
2. Description of the Related Art
Generally, a laser printer is a printing apparatus that forms a latent image on a photosensitive medium by scanning a laser beam emitted from a laser diode across the photosensitive medium and transfers the latent image to a medium such as paper, thereby reproducing an image. A laser scanning unit is an image forming device that generates a laser beam and forms an image on a photosensitive medium in a laser printer. FIGS. 1A and 1B are schematic diagrams of a conventional laser scanning unit 10. Referring to FIGS. 1A and 1B, the conventional laser scanning unit 10 includes a light source 11, a collimating lens 12, an aperture stop 13, a cylinder lens 14, a polygon mirror 15, a scanning lens 16, a mirror 17, and a photosensitive drum 18. The light source 11, the collimating lens 12, the aperture stop 13, the cylinder lens 14, the polygon mirror 15, the scanning lens 16, and the mirror 17 are housed in a separate frame 19 to prevent dust contamination and the like.
In such a structure, a light beam emitted from the light source 11 (such as a laser diode) is converted into a beam parallel with an optical axis by the collimating lens 12. The parallel beam is shaped by the aperture stop 13 and then converges in a sub-scanning direction through the cylinder lens 14, thereby forming a horizontal linear light with respect to the sub-scanning direction. Thereafter, due to the fast rotation of the polygon mirror 15, the horizontal linear light is moved in a main-scanning direction (that is, horizontally across a paper) at a uniform velocity and forms an image on the photosensitive drum 18 via the scanning lens 16 and the mirror 17. The scanning lens 16 has a consistent refractive index with respect to the optical axis and polarizes the light, which is reflected from the polygon mirror 15 at a uniform velocity, in the main-scanning direction, thereby focusing the light on the photosensitive drum 18.
In the laser scanning unit 10, the light beam passing through the scanning lens 16 must be scanned on the photosensitive drum 18 in a straight line in the main-scanning direction. However, due to assembly tolerances and aberrations, the light beam may slightly deviate in the sub-scanning direction and may not be scanned linearly along the main-scanning direction on the photosensitive drum 18, as shown in FIG. 3. Skew refers to the phenomenon that the ends A and B of a scanning line (that is, the track of a beam spot as it travels across a photoreceptor) are at different heights X1 and X2. Bow refers to a curve in the scanning line (which should be rectilinear). Skew and bow decrease printing precision, thereby degrading the quality of the picture. In a tandem type laser scanning unit used in a color laser printer, skew and bow are particularly problematic. For example, in a color laser printer using a tandem type laser scanning unit, color reproducibility greatly decreases when different amounts of bow occur on a plurality of photosensitive drums. The mirror 17 serves to compensate for skew and bow by appropriately adjusting an incident angle of the light beam.
FIG. 2 is a diagram of a conventional structure for adjusting the inclination of the mirror 17. In the conventional structure, a groove 25 is provided in the frame 19 such that a lower portion of the mirror 17 is obliquely inserted into the groove 25. A screw 22 is installed at a position on the frame 19 corresponding to an upper portion of the mirror 17. The screw 22 is rotated to appropriately adjust the mirror insertion depth and thereby adjust the inclination of the mirror 17. In addition, the mirror 17 is firmly fixed using a plate spring 30 which is fixed to the frame 19 using a screw 21.
Conventionally, to compensate for scanning distortion, the inclination of the mirror is adjusted to change the angle that the beam scans on the photosensitive drum. To more efficiently compensate for such distortion, however, the rotation angle of the mirror must be adjusted to align the angles at which a beam is scanned on the photosensitive drum. Simultaneously, the incident position of the beam on the scanning lens must be adjusted to compensate for an incident position error so that both skew and bow are efficiently compensated for at the same time. Thus, the conventional technique of compensating for scanning distortion by adjusting just the rotation angle of the mirror is not satisfactory.
Accordingly, there is a need for a new technique of simultaneously adjusting the incident angle and position of a scanning beam. In addition, there is a need for a supplementary technique that allows the mirror to be more firmly fixed.